The demodulation of modulated light at the pixel level requires the switching of a photo-generated charge current. While possible to handle both electron as well as hole currents, the common methods today use the photo-generated electron currents. This choice is largely due to the higher mobility of electrons in the semiconductor material. Some pixel architectures do the necessary signal processing based on the photo-current while others work directly in the charge domain.
All of the pixel architectures transfer photocharges through the photo-sensitive detection region to a subsequent storage area or to a subsequent processing unit. In the case of charge-domain based pixel architectures, the photocharges are generally transferred to an integration node. In order to demodulate an optical signal, the pixel has to contain at least two integration nodes that are accumulating the photo-generated charges during different time intervals.
Different pixel concepts have been realized in the last few decades. The basic principle of an in-pixel demodulation sensor was first described in U.S. Pat. No. 5,856,667 to Thomas Spirig and Peter Seitz. Each pixel of the sensor had photo-sensitive areas and transfer gates or switches associated with each of the integration sites. The sensor enables in-pixel sampling of the impinging light signal with theoretically an arbitrary number of samples.
Another similar pixel concept was described in T. Ushinaga et al., “A QVGA-size CMOS time-of-flight range image sensor with background light charge draining structure”, Three-dimensional image capture and applications VII, Proceedings of SPIE, Vol. 6056, pp. 34-41, 2006. Here, a thick field-oxide layer is used to smear the potential distribution below the demodulation gates.
A common problem of the afore-mentioned pixel architectures is the slowness of the charge transport through the semiconductor material. This decreases significantly the quality of the in-pixel demodulation process. In all pixel structures, the limiting transport speed is the step-wise potential distribution in the semiconductor substrate that is used to transport the charges through the semiconductor in lateral direction out of the photosensitive region. In those configurations, thermal diffusion dominates the transport speed instead of movement by the lateral electric drift fields resulting from the step-wise potential distribution.
More recently, new approaches to speeding the in-pixel transport of the charges have been proposed. One example by Peter Seitz and described in U.S. Pat. No. 7,498,621 B2 generates the lateral electric drift fields by passing a current through very high-resistive poly-silicon gate electrodes with the intent of producing a smoother potential distribution. Similarly, D. van Nieuwenhove et al., “Novel Standard CMOS Detector using Majority Current for guiding Photo-Generated Electrons towards Detecting Junctions”, Proceedings Symposium IEEE/LEOS Benelux Chapter, 2005, introduced another drift field pixel where a drift field in the substrate is generated by the current of majority carriers. To perform demodulation of photo-generated minority carriers, the majority current is dynamically controlled by the modulation signal.
The afore-mentioned drift field pixel concepts have drawbacks, however. First, the demodulation requires the switching of large capacitances since the whole sensitive area needs to be dynamically controlled. Secondly, an electronic current is used to generate the drift fields, resulting in significant in-pixel power consumption.
An alternative pixel concept uses a static drift field pixel. The architecture described in European Patent Application EP 1 624 490 A1, entitled “Large-area pixel for use in an image sensor”, overcomes these two problems. In contrast to the architectures mentioned before, it separates the detection and the demodulation regions within the pixel. It shows lower power consumption and, at the same time, it supports fast in-pixel lateral charge transport and demodulation.
One major application of demodulation pixels is found in real-time 3-D imaging. By demodulating the optical signal and applying the discrete Fourier analysis on the samples, parameters such as amplitude and phase can be extracted for the frequencies of interest. If the optical signal is sinusoidally modulated for example, the extraction based at least three discrete samples will lead to the offset, amplitude and phase information. The phase value corresponds proportionally to the sought distance value. Such a harmonic modulation scheme is often used in real-time 3-D imaging systems having incorporated the demodulation pixels.
Another possible application of these pixel architectures is fluorescence lifetime imaging microscopy (FLIM), where short laser pulses are used to trigger the fluorescence.
The precision of the pixel-wise distance measurement is directly proportional to the modulation frequency. Therefore, high-speed charge transfer from the photo-sensitive area to the storage site is of highest importance to enable high precision phase measurement.